Drill bits are commonly used for drilling bore holes or wells in earth formations. One type of earth-boring drill bit is the fixed-cutter bit (often referred to as a “drag” bit), which typically includes a plurality of cutting elements secured to a face region of a bit body. The bit body of an earth-boring drill bit may be formed from steel. Alternatively, the bit body may be formed from a particle-matrix composite material. A conventional earth-boring drill bit 100 is shown in FIG. 1 that includes a bit body 110 comprising a particle-matrix composite material 115. The bit body 110 is secured to a steel shank 120 having a threaded connection portion 125 for attaching the drill bit 100 to a drill string (not shown). An example of such a threaded connection portion is an American Petroleum Institute (API) threaded connection portion. The bit body 110 includes a crown 114 and a steel blank 116. The steel blank 116 is partially embedded in the crown 114. The crown 114 includes a particle-matrix composite material such as, for example, particles of tungsten carbide embedded in a copper alloy matrix material. The bit body 110 is secured to the shank 120 by way of a threaded connection 122 and a weld 124 extending around the drill bit 100 on an exterior surface thereof along an interface between the bit body 110 and the steel shank 120.
The bit body 110 further includes wings or blades 150 that are separated by junk slots 152. Internal fluid passageways (not shown) extend between a face 112 of the bit body 110 and a longitudinal bore 140, which extends through the steel shank 120 and partially through the bit body 110. Nozzle inserts (not shown) also may be provided at the face 112 of the bit body 110 within the internal fluid passageways.
A plurality of cutting elements 154 is attached to the face 112 of the bit body 110. Generally, the cutting elements 154 of a fixed-cutter type drill bit have either a disk shape or a substantially cylindrical shape. A cutting surface 155 comprising a hard, super-abrasive material, such as mutually bound particles of polycrystalline diamond, may be provided on a substantially circular end surface of each cutting element 154. Such cutting elements 154 are often referred to as “polycrystalline diamond compact” (PDC) cutting elements 154. The PDC cutting elements 154 may be provided along the blades 150 within pockets 156 formed in the face 112 of the bit body 110, and may be supported from behind by buttresses 158, which may be integrally formed with the crown 114 of the bit body 110. Typically, the cutting elements 154 are fabricated separately from the bit body 110 and secured within the pockets 156 formed in the outer surface of the bit body 110. A bonding material such as an adhesive or, more typically, a braze alloy may be used to secure the cutting elements 154 to the bit body 110.
During drilling operations, the drill bit 100 is secured to the end of a drill string (not shown), which includes tubular pipe and equipment segments coupled end to end between the drill bit 100 and other drilling equipment at the surface of the formation to be drilled. The drill bit 100 is positioned at the bottom of a well bore hole such that the cutting elements 154 are adjacent the earth formation to be drilled. Equipment such as a rotary table or top drive may be used for rotating the drill string and the drill bit 100 within the bore hole. Alternatively, the shank 120 of the drill bit 100 may be coupled directly to the drive shaft of a down-hole motor, which then may be used to rotate the drill bit 100. As the drill bit 100 is rotated, drilling fluid is pumped to the face 112 of the bit body 110 through the longitudinal bore 140 and the internal fluid passageways (not shown). Rotation of the drill bit 100 causes the cutting elements 154 to scrape across and shear away the surface of the underlying formation. The formation cuttings mix with and are suspended within the drilling fluid and pass through the junk slots 152 and the annular space between the well bore hole and the drill string to the surface of the earth formation.
Conventionally, bit bodies that include a particle-matrix composite material, such as the previously described bit body 110, have been fabricated in graphite molds using a so-called “infiltration” process. The cavities of the graphite molds are conventionally machined with a multi-axis machine tool. Fine features are then added to the cavity of the graphite mold by hand-held tools. Additional clay work also may be required to obtain the desired configuration of some features of the bit body. Where necessary, preform elements or displacements (which may comprise ceramic components, graphite components, or resin-coated sand compact components) may be positioned within the mold and used to define the internal passages, cutting element pockets 156, junk slots 152, and other external topographic features of the bit body 110. The cavity of the graphite mold is filled with hard particulate carbide material (e.g., tungsten carbide, titanium carbide, tantalum carbide, etc.). The preformed steel blank 116 may then be positioned in the mold at the appropriate location and orientation. The steel blank 116 typically is at least partially submerged in the particulate carbide material within the mold.
The mold then may be vibrated or the particles otherwise packed to decrease the amount of space between adjacent particles of the particulate carbide material. A matrix material (often referred to as a “binder” material), such as a copper-based alloy, may be melted, and caused or allowed to infiltrate the particulate carbide material within the mold cavity. The mold and bit body 110 are allowed to cool to solidify the matrix material. The steel blank 116 is bonded to the particle-matrix composite material forming the crown 114 upon cooling of the bit body 110 and solidification of the matrix material. Once the bit body 110 has cooled, the bit body 110 is removed from the mold and any displacements are removed from the bit body 110. Destruction of the graphite mold typically is required to remove the bit body 110.
The PDC cutting elements 154 may be bonded to the face 112 of the bit body 110 after the bit body 110 has been cast by, for example, brazing, mechanical, or adhesive affixation. Alternatively, the cutting elements 154 may be bonded to the face 112 of the bit body 110 during the infiltration of the molten matrix material if thermally stable synthetic or natural diamonds are employed in the cutting elements 154.
After the bit body 110 has been formed, the bit body 110 may be secured to the steel shank 120. As the particle-matrix composite materials typically used to form the crown 114 are relatively hard and not easily machined, the steel blank 116 is used to secure the bit body 110 to the shank 120. Complementary threads may be machined on exposed surfaces of the steel blank 116 and the shank 120 to provide the threaded connection 122 there between. The steel shank 120 may be threaded onto the bit body 110, and the weld 124 then may be provided along the interface between the bit body 110 and the steel shank 120.
When utilizing new particle-matrix composite materials to form bodies of earth-boring tools (e.g., a rotary drill bit), which may require techniques such as powder compaction and sintering to form the bodies of the earth-boring tools, it may not be feasible or desirable to bond a machinable metal component, such as a shank or an extension (e.g., a “crossover”), to the particle-matrix composite material 115. Bonding a machinable metal component may not be feasible or desirable before the sintering process because the sintering process may be carried out at temperatures that exceed the melting temperature of the metal from which the machinable metal component is formed. Additionally, even if the sintering temperature is below the melting temperature of the metal component, the temperatures may still be hot enough to alter the microstructure of the metal such that it no longer exhibits required or desirable physical properties. As such, it may be necessary or desirable to bond a metal component, such as a shank or an extension, to the particle-matrix composite material 115 of the crown 114 of a bit body 110 after the crown 114 has been fully sintered to a desired final density. Such processes are described in, for example, U.S. patent application Ser. No. 11/272,439, which was filed Nov. 10, 2005 and entitled “Earth-Boring Rotary Drill Bits And Methods of Manufacturing Earth-Boring Rotary Drill Bits Having Particle-Matrix Composite Bit Bodies,” the disclosure of which is incorporated herein in its entirety by this reference. Such methods may include, for example, welding or brazing a shank or an extension to the particle-matrix composite material 115 of the crown 114 of a bit body 110 after pressing and sintering a powder mixture to form the crown 114.
Shanks and extensions generally must be readily machinable to enable machining of threads or other features thereon that can be used to connect the shanks and extensions to the drill string. However, the metals from which the shanks and extensions are generally fabricated may not be compatible with the particle matrix composite material 115 of the crown 114 of the bit body 110. For example, it may be difficult or impossible to weld a metal component (e.g., a shank or an extension) to the particle-matrix composite material 115 of a bit body 110 due to differences in thermal expansion between the metal from which the metal component is fabricated and the particle matrix material 115. Such differences in thermal expansion may result in the formation of cracks in the metal component or the bit body when the metal component and the bit body 110 are welded together. As another example, the metals from which the shanks and extensions are generally fabricated may not be chemically compatible with the particle matrix composite material 115 of the crown 114 of the bit body 110. For example, as the metal component and the bit body 110 are heated during a welding or brazing process used to attach the metal component and the bit body 110 together, atomic diffusion may occur at the interface between the metal component and the bit body 110 resulting in the formation of phases of material that degrade the physical properties of the joint between the metal component and the bit body 110 (e.g., phases of material that are relatively brittle).
In view of the above, there is a need in the art for improved methods of attaching a body of an earth-boring tool comprising a particle-matrix composite material to metal components. More particularly, there is a need in the art for improved methods of attaching a particle-matrix composite bit body of an earth-boring rotary drill bit to a drill string.